GB2239363A

GB2239363A - Balanced mixer circuit

Info

Publication number: GB2239363A
Application number: GB9013278A
Authority: GB
Inventors: Tadashi Kosuga
Original assignee: Pioneer Electronic Corp
Current assignee: Pioneer Corp
Priority date: 1989-11-20
Filing date: 1990-06-14
Publication date: 1991-06-26
Anticipated expiration: 2010-06-14
Also published as: GB2239363B; GB9013278D0; JPH03160803A; US5140705A

Abstract

An RF tank circuit includes a coil L1 with a centre tap C1, it receives an input RF signal ei, and provides a balanced mixer circuit either receiving a local signal through second C8 and third C9 capacitors at first and second coil ends, respectively, with the centre tap grounded, or receiving a local signal at the centre tap (Fig 3). An impedance L5, L6 is connected between the first and second coil ends of the RF tank circuit, the impedance providing low impedance for an intermediate frequency signal, and high impedance for a high frequency signal and a local signal. A bias resistor R2 is connected between a middle connecting point of the impedance and ground. The coil of the RF tank circuit can be one of a primary (L7, Fig 2) and secondary winding L1 of an isolation transformer, with a remaining one of the primary and secondary windings being connected to an RF signal source ei, the isolation transformer preventing the leakage of a local signal to the RF signal source. The balanced mixer circuit does not require a hybrid transformer. <IMAGE>

Description

BALANCED MIXER CIRCUIT This invention relates to a balanced mixer circuit

which does not require a hybrid transformer.

A signal receiver may have a front end which includes a hybrid transformer to convert an unbalanced input RP signal into a balanced output, and to inject a local signal into the input RP signal.

. one example of a hybrid transformer arrangement is as shown in Fig. 4. In Fig. 4, reference character ei designates an input RP (radio frequency) signal source providing an RP signal. The RP signal is applied through a capacitor Cl to an RP tank circuit 1 consisting of a coil L, and a capacitor C2, and then through a capacitor C3 to an is input terminal I, of a hybrid transformer 2 serving as a balun.

The RP signal applied to the hybrid transformer 2 is of an unbalanced input type. Another input terminal 12 Of the hybrid transformer 2 is grounded through a parallel circuit consisting of a resistor R, and a capacitor C4. A local signal oscillator 3 outputs a local signal LO, which is applied through a capacitor C5 to a predetermined internal circuit connecting point P, of the transformer 2. As shown In Fig. 4, the circuit connecting point P, is connected to the connecting point of two windings which are series -connected between the input terminals I, and 12.

Impedance means 4 (or coil L2) is connected between'. the balanced output terminals 01 and 02 of the hybrid transformer. The impedance means 4 provides low impedance for an IF (intermediate frequency) signal. and high impedance for the above-described RF signal and local signal.

The balanced output terminalsi 01 and 02 Of the transformer 2 are connected to the sources of PETs Q, and Q2. respectively. The drains of the FETs are connected-to a tuning circuit 5r and the gates are grounded.

The tuning circuit 5 is a parallel circuit of a capacitor Q5 and a coil L3, which Is tuned to the IF signal. The coil L3 is electromagnetically coupled to a coil L4. The signal Induced on the-coil L4 is provided, as an IF output, at an output terminal IFout.

A supply voltage +B is applied to the center tap of the coil L3, serving as an operating voltage for the FETs Q, and Q2 forming a mixer circuit. The center tap, to which the supply voltage +B is applied, is grounded through a bypass capacitor C7.

in the above-described device, the input RF signal is mixed with the local signal, so that the difference in frequency between those two signals is extracted by the tuning circuitr thus providing an IF signal.

In the above-described device, the hybrid transformer is employed to provide a balanced mixer circuit. For use in a VHF or UHF band, the hybrid transformer must be of a transmission line type hybrid using a ferrite core. The hybrid transformer of this type is intricate in construction and high in manufacturing cost because it is formed by winding a pair of windings on a so-called 'eye core".

An object of this Invention is to eliminate the above-described difficulties accompanying the above-described arrangement. More specifically. an object of the invention is to provide a balanced mixer circuit which, as a result of eliminating the hybrid transformer, is low in manufacturing cost and small in size or occupied volume.

The above-described object of the invention has been achieved by the provision of a balanced mixer circuit in which. according to the invention, a parallel resonance coil forming an RP tank circuit has a center tap, and with the center tap grounded: a local signal is applied to the balanced output terminals of the tank circuit, respectively, through capacitors; or the local signal is applied to the center tap of the parallel resonance coil forming the tank circuit, so that the outputs of the RP tank circuit are 10 balanced.

Fig. 1 is a circuit diagram showing a first embodiment of the invention.

Fig. 2 is a circuit diagram showing a part of a second embodiment of the invention.

Fig. 3 is a circuit diagram showing a part of a third embodiment of the invention.

Fig. 4 is a circuit diagram showing a balanced mixer circuit having a hybrid transformer.

Preferred embodiments of the invention will be described with reference to the accompanying drawings. Those components which have been previously described with reference to Fig. 4 are designated by the same reference numerals or characters.

Fig. 1 shows a first embodiment of the invention, wherein an RP tank circuit 1 consisting of a coil L, and capacitor C2 has a center tap CT which is grounded. First and second ends of the coil L, are connected to f irst terminals of capacitors C$ and C9. respectively, and the remaining terminals of capacitors C$ and C9 are the balanced output terminals 01 and 02 Of the RP tank circuit.

The local signal oscillator 3 applies the local signal LO to the balanced output terminals 01 and 02, respectively, through capacitors C10 and Cl, which provide additional capacitance for the RP tank circuit.

The impedance means 4, which'provides low impedance for the IF signal and high impedance for the RP signal and the local signal, is divided Into two cQils L5 and Li, and forms a resonance circuit with the distributed capacitances CO provided between the sources and gates of the FETs Q, and Q2.

The connecting point of the coils L3 and L6 is grounded through a parallel circuit of a bias resistor R2 and a bypass capacitor C12.

The IF tuning circuit operates fundamentally in cooperation with the above-described capacitor C6, coil L6 and distributed capacitances Co.

In the first embodiment thus organized, the RP tank circuit (with the center tap grounded) provides the input RP signal to the balanced output terminals 01 and 02, and the switching elements (FETs Q, and Q2) perform a switching operation with the aid of the local signal applied to the balanced output terminals 01 and 021 so that the IF signal is extracted by the tuning circuit 5.

Fig. 2 shows a second embodiment of the invention.

In the second embodiment. a primary winding L7 is electromagnetically coupled to the coil L, forming the RP tank circuit 1. In the second embodiment. leakage of the local signal to the RP signal source ei (i.e., to the antenna side) can be prevented.

Fig. 3 shows a third embodiment of the invention. In the third embodiment, the local signal oscillator 3 applies the local signal to the center tap CT Of the coil L, f orming the RP tank circuit 1. In the third embodiment, also, the '.RP tank circuit 1 provides the balanced output terminals 01 and 02,, and the FETs Q, and Q2 perform a switching operation with i t -1 the aid of the local signal applied to the RF tank circuit, so that the IF signal is extracted by the tuning circuit 5.

As is apparent from the above description. according to the invention, the resonance coil forming the RF tank circuit has a center tap, and the local.signal is applied to both ends of the coil with the center tap grounded, or the local signal is applied to the center tap, so that the mixer circuit is in balance. Thus. the invention has eliminated use of the previously-described hybrid circuit formed by winding a pair of windings on an eye core.

Hence, the balanced mixer circuit according to the invention is low in manufacturing cost and small in size or occupied volume.

C 6

Claims

1. A balanced mixer circuit comprising:

RP tank circuit means for receiving an input RP signal and including coil means with a centre tap, and first and second coil ends having a first capacitor means connected therebetween, the RP tank circuit means providing a balanced mixer circuit receiving a local signal either through second and third capacitor means at the first and second coil ends respectively with the centre tap grounded, or at the centre tap; impedance means connected between the f irst and second coil ends of the RP tank circuit means, the impedance means providing low impedance for a predetermined intermediate frequency signal and high impedance for a predetermined high frequency signal and the local signal; bias resistor means connected between a middle connecting point of the impedance means and ground; first and second switching means connected, respectively, to the first and second ends of the RP tank circuit means, for receiving the local signal and performing a switching operation in accordance with the local signal; and tuning means connected to the first and second switching means, the tuning means being tuned to derive an intermediate frequency signal from outputs of the f irst and second switching means.

2. A balanced mixer circuit according to claim 1, wherein the local signal oscillator means is connected to the f irst and second ends of the RP tank circuit means through second and third capacitor means respectively, the centre tap being grounded, and wherein the coil means of the RP tank circuit means is one of a primary and secondary winding of an isolation transformer, with a remaining one of the primary and secondary windings being connected to an RP signal source. the isolation transformer being arranged 41 i 1 C 1 9 7 to prevent leakage of the local signal to the RF signal source.

3. A balanced mixer circuit according to claim 1, substantially as described with reference to any of the examples shown in Figures 1 to 3 of the accompanying drawings.

Published 1991 at The Patent Office, State House. 66171 High Holborn, Undon WC1R 41P. Further copies may be obtained from Sales Branch. Unit 6, Nine Mile Point Cwmfelinfach. Cross Keys. Newport. NP1 7HZ. Printed by Multiplex techniques lid. St Mary Cray. Kent.